1. Field of the Invention
The invention relates in general to electrical fuses and in particular to improved construction principles for such fuses.
It is to be noted, however, that while the present invention will be described here with reference to the particularized application of electrical fuses, the invention is not limited to such applications. Those having ordinary skill in the art and access to the teachings of this invention will recognize additional applications within the invention's scope.
2. Description of the Prior Art
FIG. 1 illustrates a cross-sectional view of a typical prior-art fuse 100. The fuse includes a tubular, ceramic body 110 and a wire fusible element 120. Fusible element 120 extends through the hollow central core 115 of fuse body 110 and includes looped ends 122 and 127 respectively folded over fuse-body ends 112 and 117. The body ends and fusible-element loops are encased within respective end caps 130 and 140 which also hold lead wires 150 and 160. Conductive contact is provided by solder 180 and 190. The entire assembly is encased within a surrounding shrink sleeve 170.
Two aspects of such fuses have now been identified as giving rise to a number of shortcomings which have been found to become especially pronounced in the installation stages of the larger assemblies in which the fuses may be employed as component elements. These difficulty-inducing aspects include first, the solder terminations 180 and 190 and second, the inherent configuration of the end caps 130 and 140. The associated consequential problems include both non-uniformities and instabilities in fuse electrical, thermal and mechanical properties. These causal factors and their consequential difficulties will now be discussed in greater detail.
FIGS. 2a and 2b illustrate the adverse consequences entailed in attempting to form a solder joint between an end cap and a fusible element juxtaposed a ceramic fuse body. FIG. 2a represents a 50X transverse cross-sectional view taken along line a--a in FIG. 1, while the enlarged 100X view of FIG. 2b focuses more specifically on the vicinity of the fusible element. The ceramic nature of the fuse body 210 does not lend itself to a capillary-type adhesion action with respect to solder 270. As a result, gaps 272 and 274 are typically encountered at the interface between body surface 215, fusible element 220 and solder 270. As is exemplified by the figure, it has been found to be not uncommon for only approximately 30% of the fusible-element surface to be effectively contacted by the adjacently-disposed solder. This unpredictable degree of contact consequentially produces a resistance across the fuse which is non-uniform from one fuse to the next. The resulting variation in inter-fuse electrical properties in turn introduces an element of uncertainty into end-use overall circuit design. It may also be noted that because such solder junctions are typically formed only after the end cap has been inserted over the looped end of the fusible wire, the contacts are not readily susceptible to quality-control inspection.
A second source of difficulty in the prior-art fuse construction concerns the positioning of the end cap around the ceramic body. This situation is schematically illustrated in the transverse cross-sectional view of FIG. 3a where the fuse body 310 of a given external diameter d.sub.1 is shown to be surrounded by the end cap 330 of internal diameter d.sub.2. Interposed between the body and cap is the fusible element 320. The consequential lack of concentricity between the body 310 and the end cap 330 is readily apparent. In practical commercial situations, this lack of concentricity is rendered more pronounced by the typical desire to form the end cap 330 of sufficiently-large internal diameter d.sub.2 so as to accommodate fusible elements 320 of different cross-sectional diameters d.sub.3 and hence of different breakdown levels.
One problem following from the lack of concentricity is a randomness in the positional distribution of end cap 330 with respect to both the body 310 and the interposed fusible element 320. This randomness introduces a further measure of non-uniformity in the electrical properties of the overall fuse.
An associated problem follows from the readily-destablized nature of the solder-based end junction when the otherwise-completed fuse is subsequently subjected to elevated-temperature operations. Such operations include the application of the shrink sleeve during fuse manufacture, as well as the additional external soldering performed when the fuse is installed within larger assemblies such as printed circuit boards. A common collateral consequence of the elevated temperatures is a remelting of the solder within the fuse end. With mechanical interlock between the various components of the conventional fuse being basically provided by only the interfacing solder and the surrounding shrink sleeve, such remelting has been found to have an adverse effect on the mechanical positioning of the end cap. The lack of concentricity illustrated in FIG. 3a, together with the absence of other available physical interlock mechanisms, can produce a post-elevated-temperature condition such as the one illustrated in FIG. 3b. Here the readily-alterable nature of the solder junction between the end cap 340 and the body 310 is seen to permit the development of an excessive tilt of end cap 340 with respect to body end 317. Among the disadvantageous consequences of an excessive tilt of this nature are the creation of shearing forces between the diagonally-disposed end-cap portion 342 and the typically-sharp corner portion 318 of fuse-body end 317. Such forces may cause either a reduction in the effective cross-sectional area of the fusible element 320 because of nicking at the 318/342 interposition point, or an actual complete severance. It may also be noted that the typically similarly-sharp corner 319 may likewise produce either nicking or actual severance when the fusible element 320 is subjected to thermally-induced tensile stresses. Such stresses include those commonly encountered in spacecraft when experiencing temperature extremes such as -55.degree. F. or lower.
It is to be further noted that another collateral consequence of such solder remelting is a physical alteration of the previously-discussed junction between the solder and the fusible element. A practically-unavoidable effect of this physical alteration is an associated additional alteration in the electrical characteristics of the junction and hence of the overall fuse.
In view of these disadvantageous properites of the prior-art fuses, a need clearly exists for significant improvements in fuse construction.